Most indoor and outdoor gardeners use at least a couple of meters routinely to help them get the most out of their efforts. Everyone wants superior production and top-quality taste and there are so many different factors affecting these two primary goals. This is why an understanding of the different meters and soil tests can be so useful.

What are the primary parameters gardeners want to know? This article provides a basic rundown of some of the main ones—moisture and nutrient levels, dissolved oxygen, salinity and electrical conductivity, total dissolved solids present in a water source, turbidity, pH, temperature, CO2 and air flow. Stay tuned for the August Maximum Yield issue for an in-depth discussion on the importance of measuring light and the use of light meters.

Moisture Meters for Gardeners

With soilless mediums such as expanded clay, grow stones, coir, rice hulls, perlite, pumice, vermiculite, sand and rockwool, the need to measure moisture is low. But indoor growers often use soil. Cycling the soil between very moist and relatively dry can be advantageous for root development and the various plant functions of respiration and nutrient uptake, but every set-up is different and a standard watering schedule will not deal with these differences. Guessing soil moisture by feeling with one’s finger can be an inaccurate process—our fingers feel cold and hot, not wet.

A moisture meter allows the grower to achieve optimal moisture levels and cycling. Throughout the growing cycle, watering requirements change quite a bit depending on the maturity and phase of the plant as well as changes in ambient air flow and temperature. Plants take oxygen in through their roots as part of the cycle to metabolize sugars to produce energy for growth, reproduction or other life processes, so the proper moisture level in the soil is critical and a meter to help with this can be vital.

Dissolved Oxygen Meters

Among the most important measurements we want to take for our nutrient solution or recirculating water is dissolved oxygen. This measurement is critical in deep water culture systems but not as important for other hydroponic systems. Because of how critical this is in DWC systems, a discussion of this parameter should be in depth—not a summary—and so is beyond the scope of this article.

Nutrient Metering and Monitoring

Nutrient levels may be the next most popular and important aspect of growing gardeners want to know and then control. Too little of any of the critical nutrients can cause a degradation in plant vigor, fruiting, root growth, basic plant health, transportation of nutrients within the plant and even taste.

High-quality, accurate meters for all the individual nutrients can be hard to find without paying a healthy price for them, but if you are serious about growing, investing in various nutrient meters will pay off. Testing kits will offer an economical approach for monitoring at least some of the most important nutrient levels. As with most things, using the proper technique is critical in achieving accurate results and successful outcomes.

The three macronutrients—nitrogen, phosphorous and potassium—are the most critical to get right. Without comprehensive knowledge of all the various nutrients in your nutrient solution or soil, trying to troubleshoot the cause of a particular symptom in your plants can be a wild goose chase. For example, chlorosis is almost always caused by a lack of iron, but deficient calcium will also cause this problem.

Electrical Conductivity and Salinity Meters

Electrical conductivity (EC) meters do not directly measure the nutrient levels in the soil or water, but the electrical conductivity, which is great for determining salinity, as salts conduct electricity. For the most part, nutrients are a form of various salts. Knowing the electrical conductivity, which indicates the salts, can give a fairly accurate reading of the concentration of nutrients in the water or nutrient solution.

Distilled water does not contain any salts and so does not conduct electricity, meaning it does not influence the EC meter’s readings, but you may need a TDS meter if you are using another water source. If you are growing in soil, an EC meter isn’t the best tool for checking nutrient levels because there are often many other non-nutrient salts in soil. Soil test kits are a better option in this case.

Total Dissolved Solids

Your raw water has an important influence on EC readings. This influence can be determined by the total dissolved solids (TDS) in the raw water going into your nutrient tank. The dissolved solids in water are for the most part salts. Some parts of the country have high TDS in the water, so when filling your nutrient tank from this type of water, the EC reading is going to be higher before you add any nutrients. To get the most accurate idea of the nutrient level using an EC meter, you would basically tare the meter reading by the factor of the raw water’s EC reading.

pH Meters

Technically, pH is the negative logarithm of the activity of the hydrogen ion. In practice, it is the measurement of the acidity or basicity of the solution or soil. Nutrient uptake by the plant is affected in a major way by the pH. Most plants prefer a pH between 5 and 6.5 (mildly acidic), and high pH will make most nutrients unavailable to plants. Too many things affect pH, so it is not practical to try to keep it at a certain level, but you can add chemicals or other nutrients that can bring your pH back to a good level. There are many accurate, inexpensive pH meters available. You should have a good idea what the pH is of your solution or soil throughout the growing cycle.

Most soil test kits use colormetrics to provide an indication of whatever nutrient or other parameter you intend to check. A color chart compares how the sample you tested looks compared to a background standard. Turbidity is a reading of suspended solids. These floating particles can and do alter the color of the sample reading, and when this occurs, you’ll need to filter them out before running the test. In most cases a simple, clean coffee filter will improve the clarity enough to give a reasonably accurate reading. You may want to rinse the filter with distilled water before using it to ensure there isn’t any pH or other influences in the filter material.

Temperature, Air Flow and Humidity

The temperature of the solution or soil and the ambient air temperature of your growroom are both important parameters to keep track of. Warm air temperatures help increase photosynthesis, transpiration and respiration. Low air temperatures reduce energy use and increase the storage of sugars.

Sustained warm temperatures can cause a plant to break dormancy and return to vigorous growth. Excessively hot air temperatures cause plant stress by increasing the demand for water through transpiration and evaporation beyond the capacity of the root system. Drought within the plant then causes desiccation and flagging, and ultimately leaf scorch, plant decline and even plant death.

Most grow lights produce heat, and that heat needs to be adequately removed from the growing area or tent. Maintaining the right air temperature is vital and though we can sense temperature with our skin, this is still an inaccurate way to determine temperature—often just entering the grow area can affect temperature.

As for soil or solution temperature, soil biology is greatly affected by temperature. If the temperature rises above 95°F, micro-organism activities slow down. This is also true when the soil/solution is too cold—below 40°F. When micro-organism activity slows down, respiration (oxidizing organic matter and producing CO2) slows down. Adequate respiration rates are needed for plant health and vigor. Improperly controlled soil or solution temperatures directly affect this.

The air needs to move within the grow area or tent to circulate the CO2 properly and make sure there are no hot areas within the tent. Air circulation is important. There are meters to check this along with the relative humidity. Some plants do poorly in dry air, while others are pretty much fine in a wide range of air moisture readings. Whether you check all three of these air readings—temperature, air flow and humidity—will depend on the plants you grow and be balanced against your budget and investment in your crop.

Carbon Dioxide Monitors

Plants take in carbon dioxide and put out oxygen during photosynthesis, which happens in the foliage, not in the roots. A plant can benefit from even moderately elevated levels of CO2 in the ambient air, but when providing additional CO2 for maximizing yield, it is critical to monitor CO2 levels. Excessive CO2 is detrimental to the health of your plants, not to mention your own health.

Having a basic idea of what types of meters are available can help you plan an initial budget or give you ideas for future additions to your tool bag. The information here is basic and only intended to help guide decisions about what meters or test kits you’ll be interested in buying. Successful growrooms are typically the result of keeping notes on previous experiences. Recording the various parameters that you measured and making slight adjustments—one at a time—can help you improve your outcomes. Growing plants is truly an application of science, get involved and enjoy!